In recent years, transistors which are used for many liquid crystal display devices and light-emitting display devices typified by flat panel displays have included a silicon semiconductor such as amorphous silicon or polycrystalline silicon and have been formed over glass substrates.
Instead of the silicon semiconductor, a technique in which an oxide semiconductor is used for transistors has attracted attention.
For example, techniques by which a transistor is manufactured using zinc oxide which is a single-component metal oxide or an In—Ga—Zn—O-based oxide which is a homologous compound as an oxide semiconductor, and is used as a switching element or the like of a pixel of a display device, is disclosed (see Patent Document 1 to Patent Document 3).